HIGH FRICTION SURFACE TREATMENT

WHAT IS HIGH FRICTION SURFACE TREATMENT? (HFST)

High Friction Surface Treatments (HFST) are pavement surfacing systems with exceptional skid-resistant properties that are not typically acquired by conventional materials.

PURPOSE OF HFSTHFST can enhance the ability of a road surface to provide pavement friction to vehicles in critical braking or cornering maneuvers. In locations such as sharp horizontal curves where vehicles may brake excessively, the road surface of standard pavements may become prematurely polished, thereby reducing the available pavement friction. This friction reduction can contribute to vehicles losing control or skidding when they speed, turn abruptly, or brake excessively. Negotiating a sharper curve demands more friction to keep vehicles on track, and that greater demand causes greater shear forces, thereby leading to even more polishing of the surface aggregate. HFST adds a skid-resistant layer to the polished areas.

HISTORY• HFSTs were first applied in the United

Kingdom during the 1960’s.• Case studies found was a 31%

decrease in crashes at intersections where the HFST was applied

• The success of a trial period near London led to using the treatment to reduce crashes and is now mandatory at certain curves, roundabouts, and intersection approaches.

• It was not until the early 2000s when the system was introduced to the United States as a safety countermeasure.

EFFECTIVENESSA report was completed for the Florida DOT in 2005 and includes reduction factors used by other state agencies. The following reductions are cited in the report:•Reductions for resurfacing a curve with a skid-resistant overlay:- 10 percent for all crashes (California)- 24 percent for all crashes (Montana)- 51 percent for wet pavement crashes (Missouri)•Reductions for overlay skid treatments:- 20 percent for all crashes on roads with ADT>5,000/lane (New York)- 13 percent for all crashes on roads with ADT<5,000/lane (New York)- 28 percent for run-off-road fatal and injury crashes (Minnesota)- 29 percent for run-off-road property damage only crashes (Minnesota)- 50 percent for wet-pavement crashes on roads with ADT>5,000/lane (New York)- 23 percent for wet-pavement crashes on roads with ADT<5,000/lane (New York)- 27 percent for all crashes (Iowa)- 42 percent for all crashes (Texas)•Reductions for general resurfacing:- 7 percent for all crashes (Arizona)- 25 percent for all crashes (Kentucky)- 25 percent for all crashes (Missouri)- 25 percent for all crashes (Oklahoma)- 25 percent for all run-off-road crashes (Arizona)- 45 percent for all wet-pavement crashes (Kentucky)- 45 percent for all wet-pavement crashes (Missouri)

From these reductions applied by various state DOTs through individual experience and research, it appears that a 20-to 30-percent reduction in all crashes and a 50-percent reduction in wet-weather crashes is a reasonable expectation for general HFST applications.

MATERIALS• Epoxy Resin• Polyester Resin• Polyurethane Resin• Acrylic Resin• Methyl Methacrylate

Binder Resin

MATERIALS• Research showed that small calcined

bauxite chips were very resistant to polishing and could be applied to the surface of an existing pavement using an epoxy resin binder.• The polished stone value (PSV) determined

from the test indicates on a scale of 0 to 100 an aggregate’s resistance to polishing.

Calcined Bauxite

EXISTING SURFACE & PREPERATIONASPHALT • For applications on new asphalt pavements, it is

recommended to wait a minimum of 30 days after paving before placement of HFST. On open graded friction course asphalt surfaces, stone mastic asphalt or pavement that has been treated with prior surface treatments, contact the manufacturer’s Technical Service department for guidance.

• Before placement of HFST on an asphalt concrete deck surface the entire deck surface should be cleaned to remove oils, dirt, rubber, curing compounds, paint carbonation, laitance, and other potentially detrimental contaminants which may interfere with the bonding or curing of the overlay. Acceptable cleaning is recognized as a surface with no oil spots, dirt or debris and the beginning exposure of coarse aggregate particles in the asphalt concrete.

• High pressure air or a vacuum should be used to remove all dust and other loose material. Brooms should not be used.

EXISTING SURFACE & PREPERATIONCONCRETE• The resin overlay should not be placed on Portland

cement concrete that is less than 28 days in place. • Any contamination of the deck or intermediate courses,

after initial cleaning, should be removed. Both courses should be applied within 24 hours following the final cleaning and prior to opening the area to traffic.

• Concrete surface should be abrasively cleaned by shot blasting or other means to remove oils, dirt, rubber, curing compounds, paint carbonation, laitance, weak surface mortar and other potentially detrimental contaminants, which may interfere with the bonding or curing of the overlay.

MEAN & METHODS (AUTO vs Mechanical)

Automated

• The applicator is continuously mixed, meter, monitor when applying the resin binder and high friction aggregate in one continuous application pass.

• The applicator vehicle is equipped with an inbuilt data management unit which is capable of producing real time data flow showing the volume of resin, the volume of aggregate applied throughout the application width.

• The automated continuous application vehicle will have continuous pumping and portioning devices that blend the polymer binder within a controlled system.

• The polymer binder is blended and mixed in the ratio per the manufacturer’s specification and the polymer binder shall be continuously applied once blended. The application vehicle should be capable of applying the minimum polymer binder spread rate.

Mechanical• Uniformly spread the resin binder onto the

surface using a serrated edge squeegee. Immediately broadcast the high friction surfacing aggregates until refusal. • The excess aggregate can be reused; the

aggregate can be reclaimed by a sweeper, but the recovered aggregate must be clean, uncontaminated, and dry.

COMMON MISTAKES & POSSIBLE FAILURE• Improper mixing of the 2-part epoxy, either

manually or by the automated equipment that did not have fail safes or monitoring devices.

• The roadway may have had significant cracking, so that over time the asphalt or concrete gave way – in these instances the epoxy and aggregate have remained completely bonded but the underlying pavement failed so it was a poor candidate for HFST.

• Many polymers such as epoxy resins will not reach their fully designed strengths when cured at lower temperatures. This can result in early loss of aggregates and premature wear in the wheel path.

LIFESPAN / DURABUILITY

HFST is an 8-12 year friction enhancing technique that supplies friction numbers (skid-testing) beyond conventional aggregates. Bauxite in particular can be expected to maintain this high friction reading for many years after installation.

Cost-BenefitsUsing an FHWA study, an economic value of one crash is estimated at $158,177 based on the average “cost” of all classifications of fatal and injury crashes.

FHWA FUNDING

CONCLUSION• Over 47,000 fatal crashes occurred on horizontal curves in the United States in the 5-

year period from 2006 to 2010. Many of these are skidding and lane or roadway departure crashes that can be attributed to low friction between the vehicles’ tires and the pavement due to polishing of the aggregate in the pavement, wet weather, speeding, and other factors.• The cited applications with over 90-percent crash reductions show that there are

clearly some locations where surface treatments are the most effective method to address safety concerns. More reasonable expectations, however, may be found in the controlled studies with lower but consistent improvements in safety, especially in reducing the number of wet-weather crashes. A 50-percent reduction in wet-weather crashes and 30-percent reduction in total crashes from HFSTs seem to be reasonable expectations. With such reductions in crashes possible, HFSTs have the potential to save thousands of lives.

THANK YOU!